Pressure sensitive adhesives (PSAs) made by photopolymerizing an alkyl acrylate and a polar copolymerizable monomer are known in the art. See, e.g., U.S. Pat. Nos. RE 24,906, 4,181,755, 4,364,972, and 4,243,500. Acrylic-based PSAs exhibit good adherence to high energy (i.e. polar) substrates.
Solvent-processed acrylic PSA compositions can be crosslinked by adding a polyfunctional crosslinking compound that reacts with a reactive group present in the polymer. Hot melt coating a PSA composition eliminates the necessity of solvent processing. To hot melt process an adhesive composition, the composition must not be crosslinked before and during the coating process; however, to achieve a PSA with balanced properties (i.e., peel and shear adhesion), the composition eventually must be crosslinked. In hot melt coating processes, this is usually done by exposure to high energy radiation (e.g., E-beam or high intensity ultraviolet radiation). Commonly, when high intensity ultraviolet radiation is used, a photoactive crosslinking species such as benzophenone is added to the composition.
Another method of photocrosslinking involves incorporating monomer units including pendent photoinitiator groups into the polymer backbone prior to coating. Such polymers can be coated and subsequently cured by conventional irradiation techniques. This process is described in U.S. Pat. No. 7,276,247 (Fansler et al.).